A. Field of the Invention
The present invention relates to a new and improved tool for removing coke from containers such as coke drums used in oil refining, and more specifically, to a new and improved decoking tool capable of automatically performing both boring and cutting operations using working fluid pressure to effect changeover between the operations.
B. Description of the Background Art
Part of oil refinery process is called delayed coking. Delayed coking involves distilling heavy oils to more valuable lighter distillate hydrocarbons (gas, gas-oil, gasoline). In this process, oil to be converted is fed into a fractionator where some of the lighter hydrocarbon constituents vaporize. The remaining heavy oils leave the bottom of the fractionator, are heated to about 900.degree. F. in a furnace and are injected into a coke drum. Breaking up of the heavy oil molecules takes place in the drum. The involved thermal cracking process in breaking up the molecules combines high temperature and pressure causing the product to break down into a lighter hydrocarbon gas, which is removed at the top of the drum, and coke which forms as a solid in the drum.
Hydraulic decoking is a method of removing solid coke from the coke drum. By this method coke is removed by means of high-impact-producing water jets. Hydraulic nozzles are incorporated in decoking tools, mounted at the end of a hollow drill stem. The tool is lowered into the coke chamber and coke is removed in two steps. First, a pilot hole is bored downward through the coke bed and second, the coke is cut and removed from the chamber.
The first step involves directing jets forward of the tool into the coke bed boring a hole of 24 to 38 inches in diameter through the solid bed of coke. For the second operation, the tool is lowered into the chamber and nozzles direct jets of water outwardly from the tool. The jets or streams of water penetrate the coke bed to the chamber wall to break the coke away from the wall and at the same time clean the side wall of the chamber. The coke bed is normally cut in layers as the cutting tool is advanced in successive steps by the operator. The water supply to the cutting nozzles is furnished by a high pressure multi-stage centrifugal pump at a remote location. The water supply is typically a large volume settling pond. During the decoking operation, the water entrains contaminants, including coke fines. After use, the water is collected and recirculated to the pond to allow entrained contaminants to settle out, with the water being reused and recirculated when it has become sufficiently clarified to permit reuse. Even so, coke fines remain in the water and are continuously recycled through the pump and decoking tool.
Hydraulic decoking systems have been in operation since the early 1930's. Since the inception of hydraulic decoking, the process has entailed separate boring and cutting operations and separate tools as exemplified by U.S. Pat. Nos. 2,245,554; 2,217,360; 2,254,848 and 2,294,719. The boring operation makes a pilot hole through the coke bed to allow the subsequent cutting operation product to exit the drum. Normally two separate tools are used. The first is a boring tool which is used to drill the pilot hole through the coke bed. The initial hole is created by a high pressure water jet. The boring tool has boring nozzles and clean-out nozzles. Protective blades are evenly placed around the tool to prevent large lumps of coke from clogging the nozzles while it is being withdrawn from the chamber. It is possible for fines to settle over the boring tool if boring is stopped for any reason before breaking through the coke bed. If fines should settle over the boring tool, the tool would be held and removal could be prevented or hampered. The purpose of the clean-out nozzles is to stir up these fines and loosen the tool allowing removal or continuation of the boring operation. The second tool is the final cutting tool. The final cutting tool includes cutting nozzles used to decoke the chamber after the pilot hole has been drilled in the chamber.
Using two tools requires complete depressurization of the boring tool, withdrawal of the tool from the coke drum, removal, replacement with the cutting tool, reentry, and repressurization between operations. These steps are extremely time- consuming and awkward.
Alternative designs providing both boring and cutting operations in one tool have been suggested but have several disadvantages. A lever operated tool disclosed in U.S. Pat. No. 3,836,434 employs a relatively complex linkage mechanism that prohibits the use of different specialized nozzles for the two operations, and requires partial withdrawal of the tool to accomplish changeover in operations.
Other cutting tools that perform both operations use a sleeve and piston valve which causes considerable disruption of the cutting water at the nozzle inlets and requires the steps of complete depressurization, complete tool removal, mechanical manipulation, attachment of motive air, reentry, and repressurization to perform the changeover operation. Tools of this type using sliding valves to direct fluid to boring nozzles and cutting nozzles are disclosed in U.S. Pat. Nos. 4,275,842 and 3,964,516 and 3,702,685. These tools require manual manipulation or biasing springs for changeover between operations. These requirements greatly complicate the use of these tools. There is a need for a decoking tool that can be changed in operation without removing the tool from the coke bed and having the capability of changeover from one operation to another by an operator at a remote location. Such a tool would substantially reduce the time required and cost incurred to clean a coke chamber.